Pre-stressed concrete members are widely used because of their superior mechanical strength for their relatively light weight and possibility of suppressing cracks. In the pre-stressed or pre-tension concrete members, reinforcement member embedded therein pre-tensioned to give a compression stress to a concrete body. By virtue of this compression stress, the concrete body is kept under a compressive state of stress while the member is being loaded or not loaded. Thus, the relatively poor tensile strength of concrete is compensated.
High strength and durability are required for the concrete and the reinforcement members used in the pre-stressed concrete members because the concrete and the reinforcement member are subjected to a constant compression stress and a tensile stress, respectively. Conventionally, steel bars are used as reinforcement members. But, as it has become clear that corrosion of steel bars plays an important role in decreasing the strength of the steel bars and the bond stress between the bars and the concrete, resulting in a gradual deterioration of the mechanical performance of the pre-stressed concrete member during a long service period.
Therefore, replacement of the reinforcement members by those made of protrusion FRP (Fiber Reinforced Plastics), made by conversion, formation and strengthening of raw materials, has been proposed recently. But in order to avoid chemical deterioration of FRP reinforcement members, the FRP reinforcement members has to be post-tensioned as follows. That is, after the concrete is solidified, FRP reinforcement members are inserted in as many sheathes previously embedded in the concrete and a post-tension force is applied to the reinforcement members by jacks, for example, so that the FRP reinforcement member does not come in direct contact with the concrete. As far as the FRP reinforcement members are used, an apparatus specially designed for giving post-tension thereto is necessary. Further, the apparatus is relatively large-scaled and expensive. The demerit becomes larger when two-dimensional post-tension has to be given to the concrete member because the number of the apparatus increases, and the apparatus have to be located in a limited space.